An X-ray computed tomography system (hereinafter, referred to as an “X-ray CT apparatus”) radiates X-rays onto a subject, detects the X-rays transmitted through the subject, and thereby obtains projection data reflecting coefficients of X-ray absorption within the subject.
Also, in recent years, X-ray CT apparatuses (hereinafter, also referred to as “photon counting CTs”) using detectors of the photon counting method have been developed. The photon counting CTs are different from conventional X-ray CT apparatuses using integral-mode detectors in that the former classify and count the energy of X-rays (photons) incident on the detectors in detecting X-rays radiated from an X-ray tube and transmitted through the subject. Thereby, it becomes possible to enable the transmittance of each part of the subject in addition to the separation of elements of the subject.
X-rays radiated from an X-ray tube include characteristic X-rays and braking X-rays. Characteristic X-rays have an energy distribution which depends on the target material, while braking X-rays have a continuous energy distribution. It should be noted that this energy distribution shows the relationship between the volume of energy and the number of photons (that is, the dose of X-rays). In photon counting CTs, braking X-rays are mainly used for making diagnoses. Braking X-rays are characterized in that the photon energy distribution varies depending on the voltage applied to the X-ray tube, with the upper limit thereof restricted by the voltage (namely, the tube voltage) applied to the X-ray tube.
On the other hand, the photon energy distribution of X-rays generated from X-ray tubes has a large deviation. In other words, the dose (namely, the X-ray intensity) differs depending on the volume of energy. Therefore, if the tube voltage is too low, sufficient photons for making a diagnosis may not be obtained in an energy region (hereinafter, referred to as an “energy band”) with a small dose. In addition, an increase in the tube voltage for obtaining photons necessary for a diagnosis may cause excess exposure in an energy band with a large dose. Therefore, with the photon counting CTs, it is required to widen the energy band covering the distribution of photons and reduce the dose of X-rays, namely, to flatten the energy distribution of photons (hereinafter, referred to as the “photon energy distribution”).